Light hydrocarbons, which broadly speaking include ethane, propane, butanes and their unsaturated analogs such as ethylene, propylene, and butene-1, for the most part are obtained from two major sources. Those sources are streams of either natural gas or refinery gas. The latter is usually obtained as a by-product of a high temperature process such as cracking or coking. As a result, a refinery gas stream is typically a more complex mixture of products than is a natural gas stream. For example, in addition to simple unsaturated compounds such as ethylene or propylene which are not normally found in significant amounts in natural gas, refinery gas may further contain diolefins and acetylenes. Typical refinery gas streams may also contain more compounds of nitrogen and sulfur and more heavy hydrocarbons (pentanes and higher) than typical streams of natural gas.
Two types of processes, absorption and distillation, have been employed to recover light hydrocarbons from streams of natural gas in which they are found. The absorption process, which is carried out at relatively high pressures and temperatures, has been adapted for use with refinery gas streams as well. However, this process has one significant drawback, in that very little light hydrocarbons of two carbon atoms, for example ethane and ethylene, are recovered by its use.
The distillation process allows for separation of light hydrocarbons including those of two carbon atoms. Since the boiling points of light hydrocarbons are at relatively low temperatures, a distillation process of necessity must be carried out at low temperatures. Attainment of the requisite low temperature has been made economical by the development of reliable turbo expanders. Using these expanders, the temperature of a gas stream at relatively high pressure can be lowered substantially by expansive cooling. Use of such expanders is economical not only because of the expansive (Joule-Thompson) effect but also because the energy equivalent of useful work can be extracted from the gas.
The combination of gas expansion with utilization of useful work and a low temperature distillation, is ideally suited for the recovery of light hydrocarbons from natural gas streams since such streams are normally available at high pressure and ambient temperature. In fact, it has become the preferred method for recovering light hydrocarbons from a natural gas stream because the expansion-distillation process is not only more efficient but also requires a lower capital investment than does an absorption plant.
Notwithstanding the fact that the distillation process is more efficient, heretofore, the recovery of light hydrocarbons from refinery gas has been exclusively by the absorption process. The reason for this has been that refinery gas is usually available at a relatively low pressure, typically atmospheric or slightly above and at a temperature usually above ambient. Therefore, even though the distillation process is the more efficient one, the requirement that the feed stream be compressed to a high pressure and pre-cooled to a point that a turbo expansion step will achieve the desired low temperature has made it economically impractical to employ the process for the recovery of light hydrocarbons from refinery gas.